1. Field of the Invention
The present invention relates to a motor, and more particularly, to a brushless motor which is required to be compact and high-power, such as those mounted in office automation equipment.
2. Description of the Related Art
As consumer demand has been increasingly shifting from a monochrome image to a color image in recent years, the office automation equipment such as copiers and laser beam printers is required to output images with high precision. Since formation of the color image on paper takes considerable time, high-speed printing is also desired. To meet these demanding requirements, a tandem system is employed to realize high-speed printing of the color image, where a plurality of photosensitive drums are arranged in series with respect to a feeding direction of the paper in formation images. It is, however, contrary to the continuing demand for downsizing the office automation equipment to provide a plurality of photosensitive drums, as it leads to an increase in size of the equipment. Hence, it is necessary to reduce the space for arranging other components in order to achieve both the high-speed printing and the downsizing of the office automation equipment. Therefore, it is required that the brushless motor, which takes a large installation space and is a main drive source of various types of mechanisms, be thinner, smaller, and high-power. Further, reduction in price of the brushless motor is also expected, against the backdrop of recent price reduction of the office automation equipment.
Configurations of conventional brushless motors will be described with reference to FIGS. 9 and 10. FIGS. 9 and 10 are cross-sectional views of exemplary conventional brushless motors, taken along an axial direction.
A brushless motor 1 shown in FIG. 9 includes a shaft 2 which rotates about a center axis, ball bearings 3 which rotatably support the shaft 2 and are spaced apart from each other, a bearing bush 4 which holds the ball bearings 3, a stator 5 which is formed by stacking a plurality of thin plates and is fixed outside the bearing bush 4 in a radial direction perpendicular to or substantially perpendicular to the axial direction, and a circuit board 6 disposed axially above the stator 5.
The fixing of the bearing bush 4 and the stator 5 to each other is performed by plastic working such as crimping, in such a way that an axially lower end portion 4a of the bearing bush 4 is deformed radially outward. Thus, a recess 5a is formed at the radially inner side of the stator 5 to receive the lower end portion 4a of the bearing bush 4. Due to the need to form the recess 5a, it is not possible to form the stator 5 by using the thin plates of the same shape. That is, both first thin plates 5b not used for forming the recess 5a and second thin plates 5c used for forming the recess 5a must be prepared to form the stator 5. There arises, therefore, a problem of lowering of production efficiency and increase in manufacturing cost, in comparison with the case of forming the stator by stacking thin plates of the same shape.
Moreover, the lower end portion 4a of the bearing bush 4 is formed by a portion of a ball bearing housing portion 4b which houses the ball bearing 3, as shown in FIG. 9. Thus, the circularity of the inner side surface of the ball bearing housing portion 4b is lowered when the lower end portion 4a is deformed. The lowering of circularity may cause such a defect both that the ball bearings 3 cannot be housed in the ball bearing housing portion 4b. And besides, when the lower end portion 4a is plastically deformed after the ball bearings 3 are housed within the ball bearing housing portion 4b, a force is applied radially inward to the outer ring of the ball bearing 3. This may cause degradation in characteristics of the ball bearing 3, or may cause such a defect that an abnormal noise occurs from the ball bearing 3 during the rotation of the brushless motor 1.
In addition, since high-performance is required for the brushless motor that is used in the office automation equipment, the bearing bush 4 and the stator 5 may separate from each other due to vibration that occurs in the stator 5, an impact applied from the outside, or the like, as long as fixing of the bearing bush 4 and the stator 5 to each other is achieved by plastic deformation such as crimping.
It is, therefore, desired to use a fixing member having a strong fixing power such as screws to fix the bearing bush 4 and the stator 5, especially in the brushless motor required to be high-power.
Next, a brushless motor in which a fixing member 8 such as a screw is used for fixing a bearing bush 4A and a stator 5A to each other is described with reference to FIG. 10.
Referring to FIG. 10, a stator placing portion 4Aa on which the stator 5A is placed is formed on the outer side surface of the bearing bush 4A. The stator 5A includes a stator core 5Ab and a coil 5Ac. The stator core 5Ab includes an annular core back portion 5Ab1 formed of a plurality of stacked thin plates and a plurality of teeth 5Ab2 extending outward from the core back portion 5Ab1 in the radial direction. The coil 5Ac is formed by winding a conductive wire around each tooth 5Ab2 of the stator core 5Ab multiple times. The conductive wire forming the coil 5Ac has a connecting wire portion (not shown) for connecting the plurality of teeth 5Ab2. The conductive wire is made of a conductor (not shown) which conducts electric current and of an insulating coating (not shown) for electrically isolating the conductor. The stator placing portion 4Aa is in contact with the entire surface of the core back portion 5Ab1. The fixing member 8 is inserted through the stator 5A such that it holds the stator 5A with the stator placing portion 4Aa, and is fastened in the stator placing portion 4Aa.
In the configuration shown in FIG. 10, the stator placing portion 4Aa is in contact with the entire surface of the core back portion 5Ab1. Thus, the connecting wire portion may be caught between the core back portion 5Ab1 and the stator placing portion 4Aa. A pinching force applied to the connecting wire portion is increased by the fastening force applied by the fixing member 8. Therefore, disconnection of the connecting wire portion or a short circuit due to separation of the insulating coating from the conductor may be caused.
If the distance in the radial direction between the stator 5A and the core back portion 5Ab1 is shortened especially by downsizing of the brushless motor, the connecting wire portion may be caught between the core back portion 5Ab1 and the stator placing portion 4A even more frequently.